Determining the time at which an etching reaction should end is very important for process control in the fabrication of micro-electronic devices. In particular, in plasma etching processes, optical emission spectroscopy has often been used to detect the products formed by the etching reaction. This technique has been useful in oxide, nitride, and metal etching operations. Heretofor, however, when the amount of exposed material on a semiconductor wafer being etched falls to about one percent of the surface area of the wafer, optical emission spectroscopy ceases to provide sufficient detection sensitivity for detecting such products. In oxide etching processes which use halocarbon gases in the generation of the etching plasma, a carbon monoxide etch product is formed and is often used to detect the time at which the process should be terminated, more commonly referred to as the end point. However, as was the case above, when very small patterns are etched on a wafer, the background levels of carbon monoxide evolved from outgassing, etching of the resist material, etching of quartz fixtures in the chamber and desorption from surfaces in the etch chamber make end point detection very difficult. In a typical manufacturing process, an end point detection apparatus consists of a monochromator tuned to monitor the etch product emission by means of a photo detector. The photo detector signal is frequently delivered to a low-pass filter or an integrator to enhance the signal-to-noise ratio of the end point signal. While low-pass filtering has been shown to effectively limit the system band-pass to that required to pass the slowly varying end point signal, response time is often unacceptable. In addition, both RC low-pass filtering and signal integration techniques are adversely affected by the fact that such measurements are centered at a direct current level, which is the region of maximum 1/f noise. Thus, both of these techniques may be adversely affected by long term drifts in the plasma generation or the optical detection systems. The above problems have been addressed in several prior patents.
Japanese Patent No. 0081929 describes the use of two optical signals to improve end point detection sensitivity in an SiO.sub.2 etching process. One optical signal is derived from a molecular CO etch product of the etching reaction which decreases at end point. The other optical signal is derived from the atomic He buffer gas in the plasma. The He atom signal increases at end point. This patent teaches that by comparing these signals throughout the course of the etching reaction, the end point signal is enhanced as the signals diverge.
Japanese Patent No. 59-61036 also describes the use of two optical signals to improve end point detection sensitivity. This patent describes the etching of Si in a CCl.sub.4 plasma. Optical signals at 400 nm and 510 nm are taught to diverge at the end point of the etching process. This patent teaches that by computing the ratio of the signals throughout the course of the etch process, the end point signal amplitude is enhanced.
Japanese Patent No. 0120674 teaches the use of phase sensitive detection to improve end point detection sensitivity. This patent further teaches the comparison of two optical signals for signal-to-noise improvement purposes. Such noise improvement is accomplished in part by the use of an external chopper or modulator in the phase sensitive detection circuit.
U.S. Pat. No. 4,491,499 to Jerde, et al shows a method and apparatus for optical emission end point detection. The technique disclosed therein measures the optical emission intensity of the plasma in a narrow band centered about a predetermined spectral line, for example, of one reactant species, and also measures the intensity of the plasma in a wide band centered about that same predetermined spectral line indicative of a background emission signal. The background signal is developed to derive a correction factor to thereby gain a more accurate indication of the intensity of the narrow band signal.
Applicants have discovered that the continuum plasma emission contains valuable information which may be detected and utilized in the end point detection process, unlike the prior systems wherein the continuum plasma emission, if recognized at all, is used to develop a correction factor so that its effects may be minimized or eliminated. Unlike the discrete atomic or molecular emission lines used to detect plasma etching end point in the prior art, the continuum plasma emission is a broad band spectral signal which arises from processes such as the radiative recombination of electrons with ions, the radiative attachment of electrons with neutral species or from the acceleration of electrons. Such a signal occurs at ultraviolet, visible, and infrared wave lengths. It is observed as a continuum between discrete emission lines. Such a continuum signal is inherent in all discharges and is sensitive to the electron density, electron temperature, and other electrical properties of the plasma which may change at end point as the nature of the exposed film changes.